This application relates to a photographic diffusion transfer method and, more particularly, to such a method wherein there are utilized silver complexing agents having advantageous properties.
The use of silver complexing agents, also referred to as "silver halide solvents", in diffusion transfer photographic methods is known in the art. In methods of this type an exposed silver halide emulsion is developed by contacting it with a photographic processing composition whereby an imagewise distribution of diffusible image forming components is formed in the unexposed and undeveloped areas of the silver halide. This imagewise distribution of image forming components is transferred to an image receiving stratum which is in superposed relationship with the silver halide emulsion layer to provide the desired transfer image.
In diffusion transfer methods wherein a silver transfer image is formed, the method is carried out in the presence of a silver complexing agent which forms a soluble and diffusible complex with undeveloped silver halide. The soluble silver complex which is formed diffuses to the superposed image receiving layer where metallic silver is deposited to provide the desired silver transfer image.
Various materials have been taught as being useful as silver complexing agents in positive-negative diffusion transfer photographic methods. Among these are thiosulfates, such as potassium and sodium thiosulfate; thiocyanates such as potassium and sodium thiocyanate; cyclic amides such as barbituric acid and uracil; 1,1-bis-sulfonyl alkanes such as are disclosed in U.S. Pat. No. 3,769,014; and alkylthioalkyl-substituted alkylsulfonylacetonitriles such as are disclosed in U.S. Pat. No. 3,975,423. These materials have been found to be suitable for performing their intended functions. Nevertheless, they are not completely satisfactory in every type of positive-negative diffusion transfer film unit wherein a silver transfer image is formed.
For example, there are known, in the art, diffusion transfer methods which are adapted to provide positive silver transfer images which may be viewed as positive transparencies without being separated from the developed negative silver image, including methods for forming additive color positive images which are viewed by projecting the transparency on a viewing surface. The film units employed in such methods are retained intact during processing and are not washed. Accordingly, the silver complexing agent, which typically may comprise from about 3 to about 15 percent by weight of the photographic processing composition, remains in the image receiving layer where the silver transfer positive image is formed. Because of factors such as the amount of the silver complexing agent present in the film unit and the physical properties of the known silver complexing agents, many of which have relatively high melting points, the known complexing agents tend to form crystals within a relatively short time, e.g., as brief as a day after formation of the image. Such crystal formation is undesirable because it detracts from the aesthetic quality of the image and in the case of projection transparencies adversely affects the quality of the projected image for viewing.
Thus, it would be desirable to provide a diffusion transfer method of this type wherein crystal formation in the positive silver transfer image is substantially or completely eliminated. The present application is drawn to such a method.